Where can a guy find a reasonably priced scope? Cheapest portable I saw, in a brief sesrch, from Fluke was $3200.

Since I work primarily in the electronics field, they just come around, often for little or nothing. For example, a technician who works for the Barns-Jewish hospital complex took home two Tektronix scopes, a very good brand, that the medical school scrapped. He gave them to me and I gave one to someone else and use the other. You might try electronics surplus stores if there are any in your area. EBAY is another possibility.

Where can a guy find a reasonably priced scope? Cheapest portable I saw, in a brief sesrch, from Fluke was $3200.

Compared to the old HP, Tek & such that needed a cart built heavy as a golf cart?

They are ALL "portable" now.

See Rigol. 4-trace, tons of bells and whistles. Google and see what folks have to say. Pick yer feature set.

I paid more for four sets of US-made HV-capable multiplier probes that the scope cost.

THIS tasking? Any-old audio-capable 'scope will do, even the USB ones that turn a laptop or hand held into one.

That said, what he NEEDS is a supplementary idler to deal with the starting issue.

- bring the present idler online.

- drop the supplementary idler(s) onto the line. I'd use two. Ten hoss, each. His 20 HP becomes either of a 20 HP, 30 HP or a 40 HP. Even a 10 HP, he wants to drop the main, leave a smaller one active for use with some other, lighter, load.

NOW start the load device.

- One or both supplementary idlers can then be dropped off. Or left on.

They don't eat much. Not even out of the f***ing about budget, "soft" starters, Vee Effing Dee and such.

- drop the supplementary idler(s) onto the line. I'd use two. Ten hoss, each. His 20 HP becomes either of a 20 HP, 30 HP or a 40 HP. Even a 10 HP, he wants to drop the main, leave a smaller one active for use with some other, lighter, load.

NOW start the load device.

/QUOTE]

It doesn't always come out that way. I had a 7 1/2 horse motor on a Reeves drive driving a Bus alternator for a variable frequency supply. My converter was a RotoPhase rated at 12 hp, starting 4 hp at a time. I had to remember to run the drive to the lowest speed when I turned it off to minimize starting load. I would start everything else in the shop first to act as idlers and could get the unit running. I never could prove that it started better with or without the extra motors. I got a three phase service soon after so I didn't investigate further but I think that the extra iron simply knocked it off the match with the starting capacitor. I would have done better adding capacitance.

It gives good 3 phase, well balanced, no issues with the generated leg. It is expensive, but so are grinders, and downtime.

You will have to find one that will convert single to 3 phase (does not fault out if only two wires are feeding it), and find out what "de-rating" is required, which is often 2:1, meaning that for a 10 HP output, you need a 20 HP VFD, in order not to do damage to the innards. Better to look at current ratings, and not HP, though.

The VFD will start slow, with good 3 phase, and can be made to accelerate at any rate you want, within reason.

The alternative is to get a MUCH bigger idler to use as the RPC. I'd not think too hard about the balance capacitors on the RPC, they are of severely limited help, since they need to be "tuned" to the idler and the load, a problem since the load is always changing. Going that way, you'd want a n idler that is 30 or 50 HP, to make sure the generated leg does not drop too low due to current..

Given the hassles of that, a suitably rated VFD is almost certainly your best choice to run the big motor. Any other motors and their controls, you may need to run from the RPC still.

- drop the supplementary idler(s) onto the line. I'd use two. Ten hoss, each. His 20 HP becomes either of a 20 HP, 30 HP or a 40 HP. Even a 10 HP, he wants to drop the main, leave a smaller one active for use with some other, lighter, load.

NOW start the load device.

It doesn't always come out that way. I had a 7 1/2 horse motor on a Reeves drive driving a Bus alternator for a variable frequency supply. My converter was a RotoPhase rated at 12 hp, starting 4 hp at a time. I had to remember to run the drive to the lowest speed when I turned it off to minimize starting load. I would start everything else in the shop first to act as idlers and could get the unit running. I never could prove that it started better with or without the extra motors. I got a three phase service soon after so I didn't investigate further but I think that the extra iron simply knocked it off the match with the starting capacitor. I would have done better adding capacitance.

Bill

You did, of course, place the appropriate "run" capacitance on EACH idler, idler side of the contactor so that each was balanced, regardless of which one(s) you are running. You did, of course, upgrade the feed so it could pass greater current?

Yes? Or maybe not...?

Start cap for the "pilot" idler isn't all that critical.

Basically all it takes to sort the OP's challenge fast and cheap is what has already BEEN SPENT, ... critical facttoid simply that it HAD BEEN working OK, just needs a bit of help with starting load...so plus -

..well...

..start with one contactor and ONE ten hoss supplementary idler. Even a 5 HP or five or 7.5 HP could do. Sort the extra caps later.

Or.... spend more money to basically trash what is paid-for to start all over with a Vee EFF(ing) DEE? And we can probably enjoy a good two to four months of hashing over why the bugger is always tripping-out?

Seriously.

KISS method. It has BEEN working as "KISS", after all. So Keep it Simple and Stupid. Just a tad stronger...

A derated VFD is very unlikely to trip out on motor current if used within reason. The trip current will be 300% or more of motor current at a 2:1 de-rate, and such setups can be made to work most of the time even with a 1:1 match on motor current.

That is something that generally happens with a closely matched VFD, a high inertia load, and accel that is either too fast or way too slow.

I am assuming this is a business, and the OP wants the grinder to just work, no hassles.

Yes some lashup of idlers might work, a bigger RPC would also work, as I mentioned, if all the details are knowm. One could argue that an RPC is simpler, but for this, the VFD seems like a direct solution.

In full disclosure...I receive a paycheck from WEG. Regardless, I have two of the WEG CFW drives which I use on my lathe and bandsaw and both have been flawless. They've also been easy to set up. These are a true industrial-level VFD and are more costly than the Chinese Ebay drives but...I guess you get what you pay for.

Below is a page from the WEG 2020 catalog. This is the CFW700 drive, in this case the single-to-three phase model. You can see that they are sold with outputs over 100 amps which is way above 10HP. You very simply connect your two single phase leads (and ground) on the input side and on the output side you connect your three leads from the grinder. That's it.

In my case, I bought a three phase input VFD and connected it for single phase input power. As JST notes above, this works if you operate in a derated mode. Why did I do this? Well....I talked to the factory expert and he said that was best for what I wanted....best being slightly less money. If I didn't have access to the factory engineer I would just buy the purpose-built single phase input VFD.

It's compact, quiet, allows 'soft' and regulated starts and stops and the motor has all three phases doing full work. Plus I can vary RPM if I want. I can also reverse rotation. Nothing else touches that.

from what research i've done apparently either vfd or soft start SHOULD accomplish the same result but unless i need variable speed (which i don't) the soft start is less expensive. was hoping someone had direct experience with soft start.

i don't want to buy either one until i am assured it would correct the problem. i'm also thinking i could get by with a smaller motor as i don't do engine blocks anymore - mostly exh manifolds and the occasional cyl head - of course that would be a swap and see thing. i do have a 5hp motor laying around but i would need pulleys, sheave, belts

Here's why a soft starter won't do you any good. Unlike a VFD, the SS is only active during starting. It takes in AC current, rectifies it to DC, redistributes it as desired, then turns it back into a new AC current that goes to your motor. But once the motor is up to speed, the SS drops out of the picture. It hands the motor off to the grid and goes to sleep.

That's fine if you have three phase power coming - but you have single phase. So...it would be starting the motor on three phase power then handing it off to a single phase source for running. Won't work. No one I know of makes a single phase input / three phase output Soft Starter. There are ways you could half-ass make that work, like a rotary converter does, but there is very little call for that in the real world.

Plus, as mentioned, the soft starter does not modulate frequency which is where the true value lies.

Another issue with a SS is that you would normally set up the SS based on the load it is driving during start. But with your grinder, I am pretty sure you don't have any sort of speed-torque curve from the manufacturer that shows what the load is. So you'd have to broad-brush and do some trial and error to tune it in .... and there's always the chance it would never work if the load is too high.

I don't like paying for stuff. I didn't like paying for my VFD. But it's done and I'm running well and I'll get over it. That's as good a deal as any.

Here's why a soft starter won't do you any good. Unlike a VFD, the SS is only active during starting. It takes in AC current, rectifies it to DC, redistributes it as desired, then turns it back into a new AC current that goes to your motor. But once the motor is up to speed, the SS drops out of the picture. It hands the motor off to the grid and goes to sleep.

That's fine if you have three phase power coming - but you have single phase. So...it would be starting the motor on three phase power then handing it off to a single phase source for running. Won't work. No one I know of makes a single phase input / three phase output Soft Starter. There are ways you could half-ass make that work, like a rotary converter does, but there is very little call for that in the real world.

Plus, as mentioned, the soft starter does not modulate frequency which is where the true value lies.

Another issue with a SS is that you would normally set up the SS based on the load it is driving during start. But with your grinder, I am pretty sure you don't have any sort of speed-torque curve from the manufacturer that shows what the load is. So you'd have to broad-brush and do some trial and error to tune it in .... and there's always the chance it would never work if the load is too high.

I don't like paying for stuff. I didn't like paying for my VFD. But it's done and I'm running well and I'll get over it. That's as good a deal as any.

it's running off a 20hp rpc now so i don't understand the 1 phase part.

having previously all the 3ph power i ever needed i have zero experience with vfd's (and my commercial electrician friend has very little exp) a question would be would a vfd replace the rpc completely as a general power source for all my 3ph machines? they only run one at a time

A motor needs to know in which direction to rotate when you go to start it. A three phase system naturally provides that. A single phase system does not - that is why all of your single phase motors have a capacitor hanging off their side. It staggers the current in one phase during start so the motor knows to start spinning. Once it is spinning, it can keep spinning in that direction. But the initial spin has to come from somewhere.

Think of driving a skid steer loader. Each handle is a phase. Until you pull back on one mote than the other, the loader doesn't know anything other than a straight line. That's what single phase power is.

What your RPC is doing is creating a 'fake' third phase that allows the motor to start. But it's not a real phase. I've not thought about RPC's in 15 years so I'm a bit foggy on the details. But from memory you are only getting about 2/3 of the motor's output...because the third leg is not really doing anything in terms of power production.

The VFD will allow a 10HP motor to run at 10HP. An RPC won't.

I don't know all the details of all your machines but generally, yes. You can use one VFD on multiple machines. In many cases you can run more than one motor on the same VFD at the same time but there are some caveats on that.

I have an RPC and it works well. But I don't have any starting issues. If I did, I'd go to a VFD without any further thought.

some of us do. I don't understand why you're so dense, but hey, it's your problem. I'll leave it up to you.

fwiw getting access to the motor to even see if that can be done will require moving 3 machines + other stuff just to get the forklift accessible then dismantling the cabinet of the machine in question

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